1. Field of the Invention
This invention relates to an improved method of operating unthrottled internal combustion engines. More specifically, this invention relates to a method for improved ignition in unthrottled internal combustion engines. This invention also relates to means for utilizing low cetane fuels in unthrottled internal combustion engines. Moreover, this invention relates to means applicable to existing diesel engines.
2. Brief Description of the Prior Art
Existing diesel engines achieve a significantly higher thermal efficiency than conventional gasoline engines in automotive use and acceptable emissions of carbon monoxide and light hydrocarbons. However, emissions levels of soot and nitrogen oxide are high and compression ratios are higher than the optimum for maximum fuel economy, especially in high speed engines. Moreover diesels are relatively hard to start as compared to automotive gasoline engines, even with electrically heated glow plugs, and require high cetane fuels. This is especially true of the lower compression diesels such as the large lower speed engines. In addition to the above cited shortcomings, conventional diesels cannot be operated at low enough compression ratios for maximum efficiency and cannot efficiently utilize low cetane fuels such as methanol and gasoline. Although in-cylinder catalysts previously proposed can both improve efficiency and reduce emissions of soot and nitrogen oxides, retrofitting of existing engines is not always economically feasible, especially with small automotive diesels. The present invention provides economical means for catalytic ignition in existing diesel engines, whether or not such engines are provided with glow plugs for use in starting.
Attempts have been made to operate unthrottled fuel injected engines at lower than diesel compression ratios. With compression ratios too low for autoignition, an ignition source such as a spark plug or a continuously operating glow plug is needed. Thus, stratified charge spark-ignited engines of various designs, both piston and rotary, have been proposed. To date, such engines have not won acceptance. For use with heavy fuels such as diesel and jet A, spark plug fouling has been a severe problem leading to the use of glow plugs. Although use of glow plugs eliminates the fouling problem with heavy fuels, a higher glow plug temperature is required for operating a low compression ratio engine than for cold starting a conventional diesel engine. This is believed to be because the compression temperature of a low compression engine is lower than that of a high compression diesel at cold start conditions. Another factor may be that the ignition temperature of hydrocarbon fuels is higher at lower pressures than at higher pressures. With the high continuous operating temperature required using conventional glow plugs in a low compression engine, typically in excess of about 1375.degree. K., plug heat losses must be minimal if plug power requirements are to be acceptable at all operating conditions. With such a low heat loss plug it has been found that not only is no electrical power required at full load operation but that plug temperatures can even exceed the temperature limits of a high temperature material such as silicon nitride. Although much larger plugs could be used to lower operating temperature to some extent, power requirements would be excessive and space might not be available. The present invention provides for operation of unthrottled internal combustion engines at lower than normal diesel compression ratios without the use of spark plugs or continuously heated glow plugs as well as improved combustion at conventional compression ratios.
Conventional spark ignition engines are typically less efficient than diesel engines in spite of operating in close approximation to the constant volume combustion Otto cycle, a more efficient cycle than the diesel cycle. This lower efficiency is believed to result primarily from the throttling losses associated with the requirement for spark ignition. Spark ignition of homogeneous charge fuel-air mixtures requires near stoichiometric fuel-air ratios for flame propagation. Thus, to control power levels the amounts of fuel and air must both be varied in step. This requires throttling of the inlet air with resultant loss of pressure energy. The requirement for operation at fuel-air ratios near stoichiometric also has the disadvantage of resulting in high emissions of nitrogen oxides. In addition, octane levels of available fuels typically limit compression ratios to below optimum levels. Operation of spark engines without throttling of the inlet air could result in an engine more efficient than the diesel, even if such engines were limited to below optimum compression ratios. The present invention provides a catalytic method for operation of in-cylinder fuel injected engines which enables such engines to more nearly approach the efficiency of an unthrottled homogeneous charge engine.
The present invention thus offers significant advantages in the operation of internal combustion engines including both improved efficiency and reduced emissions.